This invention relates to a low carrier concentration, high speed infrared detector and to a method for its manufacture. More specifically, this invention concerns itself with the development of a high speed, low carrier concentration PbSnTe photodiode that possesses a high quantum efficiency as a 10.6 .mu.m infrared detector.
Infrared photodiodes have been found to be particularly useful as receivers for 10.6.mu.m carbon dioxide laser communication systems. As a result, a considerable research effort has evolved in an attempt to develop a high quantum efficiency infrared detector with a frequency response beyond one gigahertz and an operating temperature of 77.degree. K. Infrared photodiodes with a frequency response beyond 1 GHz must be of PIN structure in order to reduce the RC time to an acceptable value. However, high efficiency PbSnTe photodiode devices require a device design in which light is brought into the p-n junction without attenuation.
In order to accomplish such a design, it has been found that a heterojunction device in which the n-type material is transparent to 8 - 14.mu.m radiation eliminates the effect of properties of the n-type layer on efficiency. For the high speed device of this invention, the n-layer serves as both a window to the incident radiation and as the wide depletion width required to achieve gigahertz bandwidths.